International Patent Application Publication No. WO 99/21824 discloses cyclic amino acids that are useful in the treatment of epilepsy, faintness attacks, neurodegenerative disorders, depression, anxiety, panic, pain, neuropathological disorders, gastrointestinal disorders such as irritable bowel syndrome (IBS) and inflammation, especially arthritis. The compounds disclosed include those of the formula: and salts thereof, in which: R is hydrogen or a lower alkyl; and R1 to R8 are each independently selected from hydrogen, alkyl of from 1 to 6 carbons, phenyl, benzyl, fluorine, chlorine, bromine, hydroxy, hydroxymethyl, amino, aminomethyl, trifluoromethyl, —CO2H, —CO2R15, —CH2CO2H, —CH2CO2R15, —OR15 wherein R15 is a alkyl of from 1 to 6 carbons, phenyl, or benzyl, R1 to R8 not being simultaneously hydrogen.
International Patent Application Publication No. WO01/28978, corresponding to U.S. patent application Ser. No. 60/160725, describes a series of novel bicyclic amino acids, their pharmaceutically acceptable salts, and their prodrugs of formula: wherein n is an integer of from 1 to 4, where there are stereocentres, each center may be independently R or S, preferred compounds being those of Formulae I-IV above in which n is an integer of from 2 to 4. The compounds are disclosed as being useful in treating a variety of disorders including epilepsy, faintness attacks, hypokinesia, cranial disorders, neurodegenerative disorders, depression, anxiety, panic, pain, neuropathological disorders, and sleep disorders.
Patent application No. EP 01400214.1 discloses the use of compounds of formula I to IV above for the prevention and treatment of visceral pain, and gastrointestinal disorders.
More recently, International Patent Application No. PCT/IB02/01146 (unpublished at the priority date of the present invention, published as WO02/85839) describes cyclic amino acids of formulae (I)-(XXV): 
wherein R1 and R2 are each independently selected from H, straight or branched alkyl of 1-6 carbon atoms, cycloalkyl of from 3-6 carbon atoms, phenyl and benzyl, subject to the proviso that, except in the case of a tricyclooctane compound of formula (XVII), R1 and R2 are not simultaneously hydrogen; for use in the treatment of a number of indications, including pain. The application discloses, as Method H, preparation of the following compounds: 
The application also discloses, as Method F, preparation of the following compounds: 
The compound of formula (70) may be prepared by Method A described in PCT/IB02/01146, illustrated here from compound (9), a known compound, see L. Y. Chen, L. Ghosez, Tetrahedron Letters, 1990, 31, 4467; C. Houge, A. M. Frisque-Hesbain, A. Mockel, L. Ghosez, J. P. Declercq, G. Germain, M. Van Meerssche, J. Am. Chem. Soc., 1982, 104, 2920. 
Compounds of formula (73) have been described in Hoffmann, H. M. R.; Ismail, Zeinhom M.; Weber, Anette. Dep. Chem., Univ. Hannover, Hannover, Fed. Rep. Ger. Tetrahedron Lett. (1981), 22(21), 1953-6.
The inventors have found that difficulties may exist in the isolation and purification of the nitro acid analogues of the cyclic amino acids described in the prior art, such as compounds of formula 71, and have discovered that the nitro acid salt derivatives of the present invention, being crystalline and having good stability, are particularly useful for improved isolation and purification.